Ubuntu Manpage: uart — driver for Universal Asynchronous Receiver/Transmitter (UART) devices

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NAME

     uart — driver for Universal Asynchronous Receiver/Transmitter (UART) devices

SYNOPSIS

     device uart

     device puc
     device uart

     device scc
     device uart

     In /boot/device.hints:
     hint.uart.0.disabled="1"
     hint.uart.0.baud="38400"
     hint.uart.0.port="0x3f8"
     hint.uart.0.flags="0x10"

     With flags encoded as:
     0x00010   device is potential system console
     0x00080   use this port for remote kernel debugging
     0x00100   set RX FIFO trigger level to ``low'' (NS8250 only)
     0x00200   set RX FIFO trigger level to ``medium low'' (NS8250 only)
     0x00400   set RX FIFO trigger level to ``medium high'' (default, NS8250 only)
     0x00800   set RX FIFO trigger level to ``high'' (NS8250 only)

DESCRIPTION

The uart device driver provides support for various classes of UARTs implementing the EIA
RS-232C (CCITT V.24) serial communications interface. Each such interface is controlled by
a separate and independent instance of the uart driver. The primary support for devices
that contain multiple serial interfaces or that contain other functionality besides one or
more serial interfaces is provided by the puc(4), or scc(4) device drivers. However, the
serial interfaces of those devices that are managed by the puc(4), or scc(4) driver are each
independently controlled by the uart driver. As such, the puc(4), or scc(4) driver provides
umbrella functionality for the uart driver and hides the complexities that are inherent when
elementary components are packaged together.

The uart driver has a modular design to allow it to be used on differing hardware and for
various purposes. In the following sections the components are discussed in detail.
Options are described in the section that covers the component to which each option applies.

CORE COMPONENT
At the heart of the uart driver is the core component. It contains the bus attachments and
the low-level interrupt handler.

HARDWARE DRIVERS
The core component and the kernel interfaces talk to the hardware through the hardware
interface. This interface serves as an abstraction of the hardware and allows varying UARTs
to be used for serial communications.

SYSTEM DEVICES
System devices are UARTs that have a special purpose by way of hardware design or software
setup. For example, Sun UltraSparc machines use UARTs as their keyboard interface. Such an
UART cannot be used for general purpose communications. Likewise, when the kernel is
configured for a serial console, the corresponding UART will in turn be a system device so
that the kernel can output boot messages early on in the boot process.

KERNEL INTERFACES
The last but not least of the components is the kernel interface. This component ultimately
determines how the UART is made visible to the kernel in particular and to users in general.
The default kernel interface is the TTY interface. This allows the UART to be used for
terminals, modems and serial line IP applications. System devices, with the notable
exception of serial consoles, generally have specialized kernel interfaces.

HARDWARE

     The uart driver supports the following classes of UARTs:

     •   NS8250: standard hardware based on the 8250, 16450, 16550, 16650, 16750 or the 16950
         UARTs.
     •   SCC: serial communications controllers supported by the scc(4) device driver.

Pulse Per Second (PPS) Timing Interface

The uart driver can capture PPS timing information as defined in RFC 2783. The API,
accessed via ioctl(2), is available on the tty device. To use the PPS capture feature with
ntpd(8), symlink the tty callout device /dev/cuau? to /dev/pps0.

The hw.uart.pps_mode tunable configures the PPS capture mode for all uart devices; it can be
set in loader.conf(5). The dev.uart.0.pps_mode sysctl configures the PPS capture mode for a
specific uart device; it can be set in loader.conf(5) or sysctl.conf(5).

The following capture modes are available:
0x00 Capture disabled.
0x01 Capture pulses on the CTS line.
0x02 Capture pulses on the DCD line.

The following values may be ORed with the capture mode to configure capture processing
options:
0x10 Invert the pulse (RS-232 logic low = ASSERT, high = CLEAR).
0x20 Attempt to capture narrow pulses.

Add the narrow pulse option when the incoming PPS pulse width is small enough to prevent
reliable capture in normal mode. In narrow mode the driver uses the hardware's ability to
latch a line state change; not all hardware has this capability. The hardware latch
provides a reliable indication that a pulse occurred, but prevents distinguishing between
the CLEAR and ASSERT edges of the pulse. For each detected pulse, the driver synthesizes
both an ASSERT and a CLEAR event, using the same timestamp for each. To prevent spurious
events when the hardware is intermittently able to see both edges of a pulse, the driver
will not generate a new pair of events within a half second of the prior pair. Both normal
and narrow pulse modes work with ntpd(8).

Add the invert option when the connection to the uart device uses TTL level signals, or when
the PPS source emits inverted pulses. RFC 2783 defines an ASSERT event as a higher-voltage
line level, and a CLEAR event as a lower-voltage line level, in the context of the RS-232
protocol. The modem control signals on a TTL-level connection are typically inverted from
the RS-232 levels. For example, carrier presence is indicated by a high signal on an RS-232
DCD line, and by a low signal on a TTL DCD line. This is due to the use of inverting line
driver buffers to convert between TTL and RS-232 line levels in most hardware designs.
Generally speaking, a connection to a DB-9 style connector is an RS-232 level signal at up
to 12 volts. A connection to header pins or an edge-connector on an embedded board is
typically a TTL signal at 3.3 or 5 volts.

Special Devices

The uart driver also supports an initial-state and a lock-state control device for each of
the callin and the callout "data" devices. The termios settings of a data device are copied
from those of the corresponding initial-state device on first opens and are not inherited
from previous opens. Use stty(1) in the normal way on the initial-state devices to program
initial termios states suitable for your setup.

The lock termios state acts as flags to disable changing the termios state. E.g., to lock a
flag variable such as CRTSCTS, use stty crtscts on the lock-state device. Speeds and
special characters may be locked by setting the corresponding value in the lock-state device
to any nonzero value. E.g., to lock a speed to 115200, use “stty 115200” on the initial-
state device and “stty 1” on the lock-state device.

Correct programs talking to correctly wired external devices work with almost arbitrary
initial states and almost no locking, but other setups may benefit from changing some of the
default initial state and locking the state. In particular, the initial states for non
(POSIX) standard flags should be set to suit the devices attached and may need to be locked
to prevent buggy programs from changing them. E.g., CRTSCTS should be locked on for devices
that support RTS/CTS handshaking at all times and off for devices that do not support it at
all. CLOCAL should be locked on for devices that do not support carrier. HUPCL may be
locked off if you do not want to hang up for some reason. In general, very bad things
happen if something is locked to the wrong state, and things should not be locked for
devices that support more than one setting. The CLOCAL flag on callin ports should be
locked off for logins to avoid certain security holes, but this needs to be done by getty if
the callin port is used for anything else.

DEPRECATION NOTICE

     The PC Card attachment of this driver is scheduled for removal prior to the release of
     FreeBSD 13.0

FILES

     /dev/ttyu?       for callin ports
     /dev/ttyu?.init
     /dev/ttyu?.lock  corresponding callin initial-state and lock-state devices

     /dev/cuau?       for callout ports
     /dev/cuau?.init
     /dev/cuau?.lock  corresponding callout initial-state and lock-state devices

HISTORY

     The uart device driver first appeared in FreeBSD 5.2.

AUTHORS

     The uart device driver and this manual page were written by Marcel Moolenaar
     <marcel@xcllnt.net>.